1. Field of the Invention
The present invention relates to a phase difference detecting apparatus for detecting phase difference with respect to rotations of two rotating axes, which rotate individually. The present invention also relates to a wheelchair using the phase difference detecting apparatus.
2. Discussion of the Background
Several methods for detecting phase differences with respect to rotations of two axes are known. A rotary encoder introduced in "A sensor applied techniques in hundred instances", pages 181-182, Akira Usami, Institute of Industrial Research, is now described. FIGS. 1(A) and (B) show a structure and circuit of an optical rotary encoder. Several holes 43 are provided at a disk 42 attached at one end of a rotating axis 41, with a certain arrangement. Beams from an LED (light emitting diode) light source 44 pass through the holes 43 when the holes 43 reach the LED light source 44. The beam passed through the holes 43 irradiates a phototransistor 45, so that the beam is detected as a voltage signal.
An increment type and an absolute type rotary encoder exist. FIG. 2 shows waveforms of the absolute type rotary encoder. As shown in FIG. 2, in the absolute type rotary encoder a whole circle of the disk 42 is binary coded, so that a detected code by the phototransistor 45 corresponds one-to-one to a rotation degree of the disk 42.
In order to obtain a phase difference with respect to rotations of two axes, two rotary encoders can be provided as shown in FIGS. 1(A) and (B). Each rotation of the two rotating axes 41 is detected, and then a difference between the detected rotations is computed.
Japanese Patent Laid-Open No. Hei 7-136218 discloses an example of an apparatus having two wheels at right and left sides thereof, i.e. a wheelchair, and to which the above-identified rotation detecting device can be applied. The wheelchair controls the wheels using the measuring result obtained by measuring rotations of the right and left side wheels. The wheelchair can be used as an only one arm drive wheelchair using electric power. Japanese Patent Laid-Open No. Hei 7-136218 describes an arm drive wheelchair having an assistance driving force generating apparatus that supplies an assistance driving force to at least one wheel, in which the rotation of the arm driven wheel is detected, and an assistance driving force corresponding to the detected result is supplied to the other wheel. In the embodiment of Japanese Patent Laid-Open No. Hei 7-136218, as shown in FIG. 3, an assistance driving force generating apparatus M is provided on the left wheel 104L side. Rotation detecting means 177L, 177R are also provided for detecting rotating speeds of the wheels 104L, 104R or displacement of the wheels 104L, 104R. Then, the rotation speed of the right wheel 104R is detected by the rotation detecting means 177R, and an assistance driving force is supplied to the left wheel 104L by the assistance driving force generating apparatus M so that the left wheel 104L can catch up with the right wheel 104R.
Referring now to FIG. 4, a speed control of the left wheel 104L is described. A motor 126 controlled by motor control means 176 generates an assistance driving force FAL, and the left wheel 104L is driven with a certain speed NL (angular velocity (.omega.L). The angular velocity .omega.L of the left wheel 104L is detected by the rotation detecting means 177L and feedback control is executed according to the detected result. That is, the angular velocity .omega.L of the left wheel 104L is compared with an angular velocity .omega.R of the right wheel 104R. Then, the rotating speed NL of the motor 126 of the assistance drive force generating apparatus M is controlled as EQU deviation.DELTA..omega.=.vertline..omega.L-.omega.R.vertline.=0.
The deviation .DELTA..omega. means deviation of the angular velocity .omega.L of the left wheel 104L to the angular velocity .omega.R of the right wheel 104R. Thus, the rotating speed NL of the left wheel 104L can be equivalent to the rotating speed NR of the right wheel 104R.
Thus, Japanese Patent Laid-Open No. Hei 7-136218 merely discloses a technique for measuring each rotating speed of the right axis and the left axis and controlling the one axis. Japanese Patent Laid-Open No. Hei 7-136218 does not disclose detection of any phase difference.
A technique controlling a rotating speed of two axes by using a detected result of the rotating speed of two axes detected by a rotation detecting means such as a rotary encoder, and an example using such a technique as in Japanese Patent Laid-Open No. Hei 7-136218, have discussed above. Hereinafter, disadvantages of such a technique recognized by the present inventors are described.
First of all, with such a technique it is necessary to provide at least two rotation detecting means, one for each wheel. Accordingly, the size of the apparatus, the weight of the apparatus, and the costs of manufacture are all increased.
Secondly, it is impossible to detect speed over a wide range regardless of a kind of sensor. For instance, when a generator type sensor is used, the sensor is not able to detect speed over a wide range. A sensor output in a low-speed range is especially low. A pulse generator type sensor also has disadvantages with respect to the pulse interval. That is, it is difficult to narrow the interval of the pulse. A function generator type sensor has also the same disadvantages as the generator type sensor.
Taking account of the above-identified disadvantages, sensitivity in a low-speed range can be improved by using speed-up gears between the rotating axes and the sensor. However, when speed-up gears are used, the voltage or frequency in a high-speed range may become too high. Thus, the detecting speed range can not be widely improved by this solution. On the other hand, sensitivity in the high-speed range can be improved by using reduction gears between the rotating axes and the sensor. However, when reduction gears are used, the voltage or frequency in the low-speed range may become too low. Thus, the detecting speed range can not be widely improved by this solution. Further, a plurality of sensors responding to different speed ranges can be provided in order to detect an object that is changing speeds over a wide range. In this case, however, the apparatus becomes large-scale and complicated.